The Ethernet Ring Protection Switching (ERPS) protocol is an industry standard and is specified within International Telecommunication Union ITU SG15 Q9, under G.8032 “Ethernet ring protection switching” (G.8032v1-2008, and G.8032v2-2010), the contents of which are incorporated by reference. ERPS specifies protection switching mechanisms and a protocol for Ethernet layer network (ETH) rings. Each Ethernet Ring Node is connected to adjacent Ethernet Ring Nodes participating in the same Ethernet Ring, using two independent links. A ring link is bounded by two adjacent Ethernet Ring Nodes, and a port for a ring link is called a ring port. The minimum number of Ethernet Ring Nodes in an Ethernet Ring is two. Two fundamental principles of G.8032 include a) loop avoidance and b) utilization of learning, forwarding, and Filtering Database (FDB) mechanisms defined in the Ethernet flow forwarding function (ETH_FF). Loop avoidance in an Ethernet Ring is achieved by guaranteeing that, at any time, traffic may flow on all but one of the ring links. This particular link is called the Ring Protection Link (RPL), and under normal conditions this ring link is blocked, i.e. not used for service traffic. One designated Ethernet Ring Node, the RPL Owner Node, is responsible for blocking traffic at one end of the RPL. Under an Ethernet ring failure condition, the RPL Owner Node is responsible for unblocking its end of the RPL (unless the RPL has failed) allowing the RPL to be used for traffic. The other Ethernet Ring Node adjacent to the RPL, the RPL Neighbor Node, may also participate in blocking or unblocking its end of the RPL. The event of an Ethernet Ring failure results in protection switching of the traffic. This is achieved under the control of the ETH_FF functions on all Ethernet Ring Nodes. An Automatic Protection Switching (APS) protocol is used to coordinate the protection actions over the ring.
G.8032v2 introduced additional features, such as: multi-ring/ladder network support; revertive/non-revertive mode after condition, that is causing the switch, is cleared; administrative commands: Forced Switch (FS), Manual Switch (MS) for blocking a particular ring port; flush FDB (Filtering database) logic, which significantly reduces amount of flush FDB operations in the ring; and support of multiple ERP instances on a single ring. With respect to multi-ring/ladder network support, G.8032 specifies support for a network of interconnected rings. The recommendation defines basic terminology for interconnected rings including interconnection nodes, major ring, and sub-ring. Interconnection nodes are ring nodes that are common to both interconnected rings. The major ring is an Ethernet ring that controls a full physical ring and is connected to the Interconnection nodes on two ports. The sub-ring is an Ethernet ring that is connected to a major ring at the interconnection Nodes. By itself, the sub-ring does not constitute a closed ring and is closed through connections to the interconnection nodes. In interconnected rings, G.8032 was not designed nor specified to gracefully handle (i.e., recover) concurrent or simultaneous multiple faults on the major ring. Whenever a [single] fault occurs on a network element blade, which is supporting ring links; there is the potential for end-to-end network “black holing” of [client] traffic being transported by G.8032 ring interconnections. This leads to increases in service unavailability and dissatisfaction of customers due to network impairments.